PPDL Picture of the Week

October 1, 2018

Horseweed

Joe Ikley, Weed Science Program Specialist, Department of Botany and Plant Pathology, Purdue University

Harvest time is often one of the best times of the year to evaluate how herbicide programs worked for season-long weed control. Weed escapes are easy to spot from the height of a combine cab, and many of us have created new four-letter words if we plug up our combines with still-green weeds. What is often more difficult to determine, is what weeds are newly emerged and utilizing sunlight that is reaching the soil surface as our crops dry down. We have already found emerged horseweed (marestail) rosettes growing in between crop rows prior to harvest (picture 1). As long as there is bare soil, and adequate moisture, we can often find horseweed germinating from late August throughout November across much of Indiana. Once our crops are out of the field, these emerged plants will capture as much sunlight and nutrients as they can until the ground freezes in order to successfully overwinter and become next year’s headache.

An early harvest this year means we will have more time to apply fall burndown herbicides for horseweed and other problematic winter annual weeds. If these applications are applied soon after harvest, this will be a year where residual herbicides can be of benefit to help keep fields weed-free until the ground freezes. Applications that are made closer to ground-freeze often do not receive as much benefit from the addition of residual herbicides, as there is less time available for new weeds to germinate this calendar year. Many effective options can be found in Table 1 of a newsletter article we published in Pest and Crop newsletter in 2017 about effective fall applied burndown herbicides (https://extension.entm.purdue.edu/pestcrop/2017/Issue21/). In general, most options that will effectively control fall-emerged horseweed will also control other winter annual weeds we find in our fields.